Generally, the top wall portion of the head of a piston used in an automobile is provided with a passage through which oil is forced to circulate in order to cool the piston during operation. In forming the circulating passage in the head, the following methods have heretofore been employed; (1) A shell core is used when the piston is cast (FIG. 3 (a)). (2) An aluminum piping 16 is introduced into the mold when the piston is cast (FIG. 3(b)). However, according to the method (1) described above, generation of gas tends to result in cavities 14 being formed in the piston. Also, according to the method (2), it is quite difficult to position the piping 16.